1. Field of the Invention
The present invention relates to a laser apparatus including a laser resonator containing a laser active medium whose vertical and lateral dimensions are different from one another in a section perpendicular to a laser optical axis.
2. Description of the Prior Art
FIG. 1 is a schematic plan view showing a conventional laser resonator disclosed in, for example, Japanese Patent Application Laid-Open No. 63-192285. FIG. 2 is a schematic sectional view showing a section perpendicular to a laser optical axis of a laser apparatus employing the laser resonator. In FIG. 2, reference numeral 20 is a 72 MHz high-frequency generator, 21 is a power matching circuit, 22 is a high-frequency cable, 23 is an insulating feedthrough, 71 and 72 are electrodes, and 73 and 74 are surfaces of the electrodes, which are polished to provide optically reflective surfaces. Further, reference numeral 75 means a discharge gap, 76 and 77 are spacers to insulate the electrodes 71, 72, and 78 is a U-shaped base. The assembly including the electrodes 71, 72 and the spacers 76, 77 is mounted on the base 78. The U-shaped base 78 is enclosed by a cover 79, and a ceramic insulator 80 is disposed between the cover 79 and the electrode 71. As shown in FIG. 1, the laser resonator includes a total reflection mirror 3 having a concave surface, and a take-out mirror 4 having a convex surface which is notched to form a notch 41a, and such a laser resonator is classified into a positive branch unstable laser resonator.
In the conventional laser apparatus constructed as set forth above, high frequency generated by the high-frequency generator 20 is applied between the electrodes 71 and 72 through the power matching circuit 21 and the high-frequency cable 22. The discharge gap 75 disposed between the electrodes 71 and 72 is filled with laser gas 1 (see FIG. 1), and the laser gas 1 is discharged and excited by the high frequency which is applied between the electrodes 71 and 72. The laser gas 1 is disposed between the total reflection mirror 3 and the take-out mirror 4 as shown in FIG. 1. Since the laser resonator including the total reflection mirror 3 and the take-out mirror 4 contains the excited laser gas 1, that is, a laser active medium 1, laser oscillation can be performed. At this time, in a plane shown in FIG. 1, the laser resonator is provided with the total reflection mirror 3 and the take-out mirror 4 to form a so-called unstable resonator. Further, the laser resonator can serve as a so-called waveguide type resonator in a direction perpendicular to the plane. In other words, the resonator is an unstable/waveguide type hybrid resonator. A laser beam is emitted externally to the laser resonator from the notch 41a of the take-out mirror 4. Assuming that the distance between the electrodes 71 and 72 is 2 mm, and distances between edges of the electrodes 71, 72 and an edge of the take-out mirror 4 including the convex surface mirror are 2 mm. It is possible to derive a square beam having each side of about 2 mm from the notch 41a. It has been considered that the beam becomes a substantially circular beam if the beam is separated from the laser resonator by a predetermined distance.
Since the conventional laser apparatus is constructed as set forth above, optical energy having weak intensity is generated due to diffraction of light even on the outside of a ray 521 of a geometrically and optically outermost portion. The phase of the light largely fluctuates so that the light may cause disturbance of the laser beam external to the laser resonator when the light is emitted and propagated out of the laser resonator. Hence, the laser beam derived from the laser resonator is a laser beam which is not symmetrical with respect to an unstable direction. Consequently, there is a problem in that the laser beam has a deformed circular form rather than a completely circular form at a position apart from the laser resonator by the predetermined distance.